ATLANTA — First lady Michelle Obama debuted on the 2014 campaign trail Monday in Georgia, where Senate candidate Michelle Nunn is working to pick up a key seat for Democrats. The open Senate seat is a critical battleground in the national fight for control of the Senate, in which Republicans must gain six seats to win the majority. Businessman David Perdue has relentlessly tried to tie Nunn to President Barack Obama, who did not win Georgia in 2008 or 2012.